63 research outputs found
Divergence-free approach for obtaining decompositions of quantum-optical processes
Operator-sum representations of quantum channels can be obtained by applying
the channel to one subsystem of a maximally entangled state and deploying the
channel-state isomorphism. However, for continuous-variable systems, such
schemes contain natural divergences since the maximally entangled state is
ill-defined. We introduce a method that avoids such divergences by utilizing
finitely entangled (squeezed) states and then taking the limit of arbitrary
large squeezing. Using this method we derive an operator-sum representation for
all single-mode bosonic Gaussian channels where a unique feature is that both
quantum-limited and noisy channels are treated on an equal footing. This
technique facilitates a proof that the rank-one Kraus decomposition for
Gaussian channels at its respective entanglement-breaking thresholds, obtained
in the overcomplete coherent state basis, is unique. The methods could have
applications to simulation of continuous-variable channels.Comment: 18 pages (8 + appendices), 4 figs. V2: close to published version,
dropped Sec.VI of v1 to be expanded elsewher
Heisenberg-limited eavesdropping on the continuous-variable quantum cryptographic protocol with no basis switching is impossible
The Gaussian quantum key distribution protocol based on coherent states and
heterodyne detection [Phys. Rev. Lett. 93, 170504 (2004)] has the advantage
that no active random basis switching is needed on the receiver's side. Its
security is, however, not very satisfyingly understood today because the bounds
on the secret key rate that have been derived from Heisenberg relations are not
attained by any known scheme. Here, we address the problem of the optimal
Gaussian individual attack against this protocol, and derive tight upper bounds
on the information accessible to an eavesdropper. The optical scheme achieving
this bound is also exhibited, which concludes the security analysis of this
protocol.Comment: 10 pages, 6 figure
Experimental Proof of Quantum Nonlocality without Squeezing
It is shown that the ensemble where is a Gaussian distribution of finite variance and is a
coherent state can be better discriminated with an entangled measurement than
with any local strategy supplemented by classical communication. Although this
ensemble consists of products of quasi-classical states, it exhibits some
quantum nonlocality. This remarkable effect is demonstrated experimentally by
implementing the optimal local strategy together with a joint nonlocal strategy
that yields a higher fidelity.Comment: 4 pages, 2 figure
Optimality of Gaussian Attacks in Continuous Variable Quantum Cryptography
We analyze the asymptotic security of the family of Gaussian modulated
Quantum Key Distribution protocols for Continuous Variables systems. We prove
that the Gaussian unitary attack is optimal for all the considered bounds on
the key rate when the first and second momenta of the canonical variables
involved are known by the honest parties.Comment: See also R. Garcia-Patron and N. Cerf, quant-ph/060803
Boson Sampling in Low-depth Optical Systems
Optical losses are the main obstacle to demonstrating a quantum advantage via
boson sampling without leaving open the possibility of classical spoofing. We
propose a method for generating low-depth optical circuits suitable for boson
sampling with very high efficiencies. Our circuits require only a constant
number of optical components (namely three) to implement an optical
transformation suitable for demonstrating a quantum advantage. Consequently,
our proposal has a constant optical loss regardless of the number of optical
modes. We argue that sampling from our family of circuits is computationally
hard by providing numerical evidence that our family of circuits converges to
that of the original boson sampling proposal in the limit of large optical
systems. Our work opens a new route to demonstrate an optical quantum
advantage.Comment: 11 pages, 6 figure
Continuous variable quantum key distribution with two-mode squeezed states
Quantum key distribution (QKD) enables two remote parties to grow a shared
key which they can use for unconditionally secure communication [1]. The
applicable distance of a QKD protocol depends on the loss and the excess noise
of the connecting quantum channel [2-10]. Several QKD schemes based on coherent
states and continuous variable (CV) measurements are resilient to high loss in
the channel, but strongly affected by small amounts of channel excess noise
[2-6]. Here we propose and experimentally address a CV QKD protocol which uses
fragile squeezed states combined with a large coherent modulation to greatly
enhance the robustness to channel noise. As a proof of principle we
experimentally demonstrate that the resulting QKD protocol can tolerate more
noise than the benchmark set by the ideal CV coherent state protocol. Our
scheme represents a very promising avenue for extending the distance for which
secure communication is possible.Comment: 8 pages, 5 figure
Improvement of continuous-variable quantum key distribution systems by using optical preamplifiers
Continuous-variable quantum key distribution protocols, based on Gaussian
modulation of the quadratures of coherent states, have been implemented in
recent experiments. A present limitation of such systems is the finite
efficiency of the detectors, which can in principle be compensated for by the
use of classical optical preamplifiers. Here we study this possibility in
detail, by deriving the modified secret key generation rates when an optical
parametric amplifier is placed at the output of the quantum channel. After
presenting a general set of security proofs, we show that the use of
preamplifiers does compensate for all the imperfections of the detectors when
the amplifier is optimal in terms of gain and noise. Imperfect amplifiers can
also enhance the system performance, under conditions which are generally
satisfied in practice.Comment: 11 pages, 7 figures, submitted to J. Phys. B (special issue on Few
Atoms Optics
Effect of Intensity Modulator Extinction on Practical Quantum Key Distribution System
We study how the imperfection of intensity modulator effects on the security
of a practical quantum key distribution system. The extinction ratio of the
realistic intensity modulator is considered in our security analysis. We show
that the secret key rate increases, under the practical assumption that the
indeterminable noise introduced by the imperfect intensity modulator can not be
controlled by the eavesdropper.Comment: 6 pages, 5 figures. EPJD accepte
Continuous Variable Quantum Cryptography using Two-Way Quantum Communication
Quantum cryptography has been recently extended to continuous variable
systems, e.g., the bosonic modes of the electromagnetic field. In particular,
several cryptographic protocols have been proposed and experimentally
implemented using bosonic modes with Gaussian statistics. Such protocols have
shown the possibility of reaching very high secret-key rates, even in the
presence of strong losses in the quantum communication channel. Despite this
robustness to loss, their security can be affected by more general attacks
where extra Gaussian noise is introduced by the eavesdropper. In this general
scenario we show a "hardware solution" for enhancing the security thresholds of
these protocols. This is possible by extending them to a two-way quantum
communication where subsequent uses of the quantum channel are suitably
combined. In the resulting two-way schemes, one of the honest parties assists
the secret encoding of the other with the chance of a non-trivial superadditive
enhancement of the security thresholds. Such results enable the extension of
quantum cryptography to more complex quantum communications.Comment: 12 pages, 7 figures, REVTe
An improved two-way continuous-variable quantum key distribution protocol with added noise in homodyne detection
We propose an improved two-way continuous-variable quantum key distribution
(CV QKD) protocol by adding proper random noise on the receiver's homodyne
detection, the security of which is analysed against general collective
attacks. The simulation result under the collective entangling cloner attack
indicates that despite the correlation between two-way channels decreasing the
secret key rate relative to the uncorrelated channels slightly, the performance
of the two-way protocol is still far beyond that of the one-way protocols.
Importantly, the added noise in detection is beneficial for the secret key rate
and the tolerable excess noise of this two-way protocol. With the reasonable
reconciliation efficiency of 90%, the two-way CV QKD with added noise allows
the distribution of secret keys over 60 km fibre distance.Comment: 12 pages, 6 figure
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